Programmable ultrafast optical pulse shapers are ubiquitous, with applications ranging from simple dispersion control to carefully shaped laser pulses used for coherent control of quantum dynamics, femtosecond microscopy and spectroscopy, nonlinear fiber optics, high harmonic generation, etc. While many applications utilize spectral phase-only shaping for simplicity, one also needs control over amplitude to access a complete range of temporal shapes, for example, square and sinc pulses. Such control in the standard zero-dispersion Martinez stretcher has been implemented with volume Bragg gratings written with an acousto-optic modulator (AOM) crystal, pairs of liquid crystal spatial light modulators (LC-SLMs) and, more recently, by a two-dimensional (2D) LC-SLM having a blazed phase grating in the direction lacking spectral dispersion. A complementary technology, acousto-optic programmable dispersion filters (AOPDFs), also provides spectral phase and amplitude control.
Diffractive effects have been used in AOM devices to shape both spectral phase and amplitude by Bragg diffraction of the incident light at the Fourier plane, and reconstructing the first-order diffracted beam into a shaped pulse. Thus, the acousto-optic device throughput is fundamentally limited by the first-order diffraction efficiency, reported to be ˜50% at 620 nm in an experimental device. The optical bandwidth of AOM shapers is limited to a single octave since beyond this, higher order diffracted beams will overlap. AOMs can be rapidly reconfigured between pulses, but finite acoustic wave velocity places an upper limit on the repetition rate of shaped pulses. While this is well-suited for amplified pulses, a pulse picker must be employed in the typical cases of multi-MHz ultrafast oscillator pulse trains.
Independent phase and amplitude control has been achieved using two, LC-SLM arrays. However, two modulators are required which doubles the circuitry, increases the bulk and requires accurate alignment of the two modulators. Additionally, the requirement for two polarizers increases losses and dispersion. More recently, a zero-order apparatus has been reported for complex-spectral phases and amplitude modulation. Eugene Frumker and Yaron Silberberg in “Phase and Amplitude Pulse Shaping with Two-Dimensional Phase-Only Spatial Light Modulators,” J. Opt. Soc. Am. B 24, 2940-2947 (2007) describe a two-dimensional spatial light modulator disposed in the Fourier plane of a cylindrical Fourier lens which ensures that each wavelength component, while focused in the horizontal direction at the Fourier plane, extends over a few millimeters in the vertical direction for independent modulation of both spectral phases and amplitudes as a result of a vertical phase grating being formed in each column of the two-dimensional spatial light modulator.